The purpose of the project is to continue the development of hermetically sealed pacemakers using a single high capacity rechargeable mercury-silver battery (not nickel-cadmium). The fixed rate version has paced 5 dogs in complete heart block for from 1 to 1 1/2 years each without any pacemaker problems. Clinical testing was begun several weeks ago. These pacemakers will run for 3 1/2 years without recharging and require approximately one hour of charging per month or 12 hours per year to maintain the battery fully charged. Estimation of the total pacing life of the unit with periodic recharging, is greater than 20 years. Further support is needed to: 1) continue chronic bench testing of the mercury-silver batteries to determine their potential functional life (we currently have batteries functioning in a simulator with over 40 years of life with accelerated tests and with actual energy drains in excess of five years); 2) chronically maintain the dogs that presently have fixed rate rechargeable pacemakers implanted; 3) continue development and implantation in animals of a new, unique ventricular programmed pacemaker utilizing state of the art circuitry that has low battery drain and is exceptionally safe in the presence of electromagnetic interference fields and ventricular premature beats; 4) continue studies in the development of a new improved cardiac pacemaker electrode system with optimal and separate sensing and pacing functions for use with all programmed pacemakers. The overall aim is to continue a comprehensive pacemaker development program involving the participation of surgeons, engineers, and veterinarians to produce universally available, inexpensive, long-life, reliable cardiac pacemakers and electrodes in order to decrease or eliminate reoperative pacemaker implantation.